1. Field of the Invention
The present invention relates to a high-capacity nonaqueous secondary battery comprising a high-density negative electrode with a high capacity and a high rate of utilization, and also to electronic equipment comprising said battery.
2. Prior Art
Nonaqueous secondary batteries represented by lithium ion secondary batteries have been increasingly used, since they have a high capacity, a high voltage, a high energy density and a high output. Also, studies with the aim of increasing the capacity and charging voltage of batteries have been made, and it is expected that the electrical energy discharge will need to be increased in batteries.
In order to increase the capacity of nonaqueous secondary batteries using the same active materials, the following measures are contemplated: (1) increasing the rate of utilization or the capacity of the active material, (2) raising the content of the active material in an electrode, and (3) increasing the density of an electrode. With regard to (1), a graphite negative electrode has a high capacity, which approaches the theoretical value of 372 mAh/g. With regard to (2), the binder content can be decreased to about 2% while the content of the active material is increased to 98%, which are close to the limits.
On the other hand, with regard to (3), the theoretical true density of graphite is 2.1 to 2.2 g/cm3, while the actual electrode density of negative electrodes are about 1.6 g/cm3, and thus it is considered that a more likely path which will end up improving the capacity is in increasing the negative electrode density.
However, when graphite with a higher crystallinity which achieves a higher capacity is used, the rate of utilization and the load characteristic of the electrode decreases, as the electrode density is increased, and thereby a problem occurs in that the increase in the density does not lead to a high energy density.
The causes for the decrease in the rate of utilization of the electrode in accordance with increasing the density of an electrode have been analyzed to solve the above problem. The first cause was thought to be that the increase in the electrode density makes it difficult for an electrolyte to enter-into the electrode. With the increase in the density of a negative electrode, spaces among active material particles gradually decrease. Particularly, when the electrode density becomes 1.70 g/cm3 or higher, the electrolyte hardly enters into the active material so that, in some parts, the surface of the active material are not in contact with the electrolyte, and thus the rate of utilization of the active material decreases in those parts. In order to improve this, it is required that the spaces into which the electrolyte can enter are secured in the active material.
As patent publications disclosing the diameter and volume of micropores (holes) of an electrode, the following patent publications are known:
JP-A-6-267590 discloses a secondary battery which comprises graphitic particles as an active material and defines the porosity of a negative electrode and the ratio of a volume occupied by pores having certain diameters to a total pore volume as well as the solvent and electrolyte of an organic electrolyte to be used.
JP-A-10-050298 defines the density of a negative electrode mixture and the ratio of a volume occupied by pores having certain diameters or more to a total pore volume.
JP-A-9-027315 controls a pore volume, a porosity, an average pore diameter, and the like by mixing carbon materials having different shapes at a specific ratio.
JP-A-9-129232 defines the ratio of a volume occupied by pores of a negative electrode having diameters in a certain range to a total pore volume, and an average pore diameter.
JP-A-06-267590 describes that “when a packing density is high and the sum of the volumes of pores having a diameter of 0.5 μm or more is small, a capacity is low”, and JP-A-10-050298 describes that “when an average diameter in a pore distribution is less than 0.5 μm, holes required for diffusing lithium ions cannot be secured among the graphitic particles, so that a capacity maintenance factor during discharging greatly decreases.” Therefore, it is expected that pores of less than 0.5 μm in size make it difficult to obtain a discharge.
With regard to a process of producing a negative electrode such that a negative electrode density is 1.2 g/cm3 or more and the sum of volumes of pores in a negative electrode mixture having a pore diameter of 0.5 μm or more occupies 80% or more of the total pore volume of the negative electrode, JP-A-10-050298 describes that a sublimable substance such as naphthalene is contained in the negative electrode mixture so as to be gasified and vaporized with decomposition by heating, and that a solid component soluble in an electrolyte is added to the negative electrode mixture in an amount of 1 to 40% based on graphite so as to dissolve the solid component in the electrolyte.
In the former case, it is thought that the residual sublimable substance in the electrode may cause self-discharge and resulting in the expansion of the battery during storage at high temperatures. In the latter case, when the packing density and porosity of the negative electrode are adjusted by the addition of the solid component soluble in an electrolyte to the negative electrode so as to dissolve the above-mentioned solid component, the wettability of the electrode is increased since an alkali metal salt or an alkaline earth metal salt previously contained in the electrode is eluted from the electrode into the electrolyte during the contact of the electrolyte with the electrode so as to improve the penetrability of the electrolyte and such a salt acts as an electrolyte, and additionally voids formed after the elution function as passages of ions (see JP-A-06-295741).